Hydraulic fracturing is a common stimulation technique used to enhance production of fluids from subterranean formations. Hydraulic fracturing is typically used to stimulate low permeability formations where recovery efficiency is limited.
During hydraulic fracturing, a fracturing fluid is pumped at high pressures and high rates into a wellbore penetrating a subterranean formation to initiate and propagate a fracture in the formation. Well productivity depends on the ability of the fracture to conduct fluids from the formation to the wellbore. The treatment design generally requires the fluid to reach maximum viscosity as it enters the fracture which affects the fracture length and width. The requisite viscosity is typically obtained by the gellation of viscosifying polymers and/or surfactants in the fracturing fluid. The gelled fluid is typically accompanied by a proppant which results in placement of the proppant within the produced fracture.
Once the fracture is initiated, subsequent stages of fracturing fluid containing proppant are pumped into the created fracture. The fracture generally continues to grow during pumping and the proppant remains in the fracture in the form of a permeable “pack” that serves to “prop” the fracture open. Once the treatment is completed, the fracture closes onto the proppants which maintain the fracture open, providing a highly conductive pathway for hydrocarbons and/or other formation fluids to flow into the wellbore.
Filtrate from the fracturing fluid ultimately “leaks off” into the surrounding formation leaving a filter cake comprised of fluid additives. Such additives, including the viscosifying polymers and/or surfactants used to provide fluid viscosity, are typically too large to penetrate the permeable matrix of the formation. Recovery of the fracturing fluid is therefore an important aspect to the success of the fracturing treatment.
Recovery of the fracturing fluid is normally accomplished by reducing the viscosity of the fracturing fluid (breaking) such that the fracturing fluid flows naturally from the formation under the influence of formation fluids and pressure. Conventional oxidative breakers react rapidly at elevated temperatures, potentially leading to catastrophic loss of proppant transport. Encapsulated oxidative breakers have experienced limited utility at elevated temperatures due to a tendency to release prematurely or to have been rendered ineffective through payload self-degradation prior to release. Thus, the use of breakers in fracturing fluids at elevated temperatures, i.e., above about 120-130° F., typically compromises proppant transport and desired fracture conductivity, the latter being measured in terms of effective propped fracture length. Improvements in hydraulic fracturing techniques are required in order to increase the effective propped fracture length and thereby improve stimulation efficiency and well productivity.
Recently, fluids (such as water, salt brine and slickwater) which do not contain a viscosifying polymer have been used in the stimulation of tight gas reservoirs as hydraulic fracturing fluids. Such fluids are much cheaper than conventional fracturing fluids containing a viscosifying polymer and/or gelled or gellable surfactant. In addition, such fluids introduce less damage into the formation in light of the absence of a viscosifying polymer and/or surfactant in the fluid.
The inherent properties of fluids not containing a viscosifying polymer, such as slickwater, present however several difficulties. Foremost, such fluids provide poor proppant transport as well as poor fluid efficiency (leakoff control). Further, the low viscosity of fluids like water, salt brine and slickwater makes it difficult, if not impossible, to generate the desired fracture width. This affects the requisite conductivity of the propped fracture as proppant placement in the fracture is often not possible.
To address such limitations, “hybrid” fracturing techniques have evolved wherein a conventional gelled and/or crosslinked fracturing fluid is used as a pad fluid which precedes the introduction of a proppant laden slickwater slurry. The relatively high viscosity gelled fluid provides increased fracture width and improved fluid efficiency, thereby mitigating the limitations of slickwater. Unfortunately, however, viscosifying polymers and surfactants used in such viscosified fluids form filter cakes on fracture faces which cause conductivity damage. Since the concentration of proppant in fracturing fluids free of viscosifying polymer and viscoelastic surfactant is low and results in propped fracture widths typically no greater than one layer of proppant (±0.5 mm), any effective fracture width lost to the deposition of a filter cake often has catastrophic consequences on fracture conductivity.
Alternative hydraulic fracturing methods have therefore been sought which increase the effective propped fracture length of created fractures and which enhance fracture conductivity. Alternative methods have been particularly sought for fracturing using fluids which are free of viscosifying polymers.